Wherever the restless, unsteady mind wanders off, from each of those places let one rein it in and bring it under control within the self alone.
Explanation
Krishna is teaching Arjuna how to practise meditation (dhyāna-yoga, the discipline of union through focused attention). Having said to withdraw the mind gradually, he now gives the method: each time the mind wanders, notice it, pull it back, and settle it in the self (ātman). Commentators differ on whether "the self" here means the inner Self or simply one's own self-command, but both read it as steady, patient redirection.